


Superior M(ass)

by modestfuckup



Series: Get Beached 2020 [11]
Category: Teen Wolf (TV)
Genre: Dead Dove: Do Not Eat, Derek is kinky but doesn't know it, Don't Like Don't Read, Fedder!Derek, Get Beached 2020, M/M, Pear shaped!Stiles, Weight Gain, chubby!Stiles, feedee!stiles, normal!derek, oblivious!Stiles
Language: English
Status: Completed
Published: 2020-07-28
Updated: 2020-07-28
Packaged: 2021-03-05 20:42:29
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,651
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/25571515
Author URL: https://archiveofourown.org/users/modestfuckup/pseuds/modestfuckup
Summary: This is an entry for Get Beached 2020, a chubby!Kink summer event.Challenge week (2500+)Stiles needs to gain some weight and Derek figures out a way to help. The results are exceptional if Derek says so himself.
Relationships: Derek Hale/Stiles Stilinski
Series: Get Beached 2020 [11]
Series URL: https://archiveofourown.org/series/1811458
Kudos: 69





	Superior M(ass)

**Author's Note:**

> Hello. Happy Tuesday! 
> 
> I don't have too much to say for this one, but I hope you all like it!

Derek waits for Stiles to finish his doctor’s appointment dutifully in the waiting room. He fucks around on his phone and checks the time enough that a minute feels like 5. 

“Alright, thanks doc,” Stiles’ voice comes from the door that leads back to the patient rooms. 

“I’m sure it’s nothing to worry about, try altering your diet to something with a higher caloric intake.” 

“I’ll try,” the dejected sound from Stiles makes Derek think he didn’t get the news he wanted. 

Stiles has always struggled to keep weight on. When they were in high school, Stiles was a walking stick, and people always teased that if he turned to the side, he’d disappear. Since they started college, Stiles has _lost_ weight rather than gained the expected freshman fifteen. 

Derek stands and meets Stiles at the counter where he schedules another appointment for a month from now. His weight was so low, Stiles’ dad suggested seeking medical assistance. 

“So?” Derek asks, grabbing Stiles hand as they exit the office. 

“He doesn’t know. He said the same old speal about stress in conjunction with my ADHD med might be fucking with my metabolism, but he says it usually has the _opposite_ effect. At the end, he just suggested I eat more,” Stiles was frustrated, Derek could tell. That morning Stiles weighed 108 pounds. It was pathetic. 

Derek’s camaro is parked outside, and the two get in. “Do you wanna get lunch?” 

“Sure.”

They eat out _a lot,_ with no real results to show for it. The next week, Stiles shows only a half pound gain despite the multiple meals coming from a drive thru window. 

Derek decides he needs to start doing more. He does some research and tries to start cooking more meals that are rich and full of fat, carbs, and calories. They keep snacks stashed around their apartment, and Stiles always has a root beer or other sweet drink in his hand. Slowly they start moving in the right direction, Stiles showing a two pound gain after a week. It’s still not enough though. 

Derek read some stuff about protein shakes helping gain weight, so he adds it to the grocery list. He stops in the cereal aisle and loads up on PopTarts because those are quick snacks Stiles can eat in class. The protein shakes and meal replacers are across the way and Derek takes a moment to analyse the shelves for something, _anything,_ that might prove to be helpful.

There are various bottles, all promising maximum muscle gain or slimming down qualities. Derek nearly gives up, having half expected this lead to be a disappointment as well, but he crouches down to the bottom shelf, filled with food bars and other smaller containers. There’s a bottle with ‘Superior Mass’ written in big green letters and Derek picks it up.

_Weight GAIN supplement. Superior Mass promotes weight gain in all tests and trials!_

Derek glances at the nutrition facts on the back, and scans the instructions. It seems like a normal protein shake powder. There is a testimony on the back of the bottle: “Superior Mass helped me gain weight in all the right places!”

At this point, Derek is willing to try it. The bottle was the only one on the shelves that was promising weight _gain_ , and he really didn’t want to go to another store or order something online. He chucks the bottle into the cart and moves to the next aisle to stock up on Oreos and Chips Ahoy. 

Derek makes a breakfast shake for Stiles with the new powder. He follows the instructions, which call for water, he substitutes full fat milk and gives it a taste. It’s not overly sweet, a good thing given it’s 8am, but it is a bit chocolatey, which Derek knows Stiles will like. 

Stiles rushes out the door, running late as always to his early morning lecture. He grabs the breakfast burrito Derek made, his sack of snacks, and the shake. Derek hopes it’s enough to get him to lunch. He gets ready for his own classes. 

The routine goes on for a couple weeks. Their weigh in show that _something_ has worked, Stiles showing a 5 pound gain. Stiles says his doctor is impressed with the results and tells Stiles to keep it up. So they do. 

They continue to eat out, and when they do make food at home it’s often filled with a lot of extra things like butter or cream to make it richer. As a result, Derek notices his own pants tightening a bit and makes a point to work out harder while he’s at the gym. The weeks prove that Stiles _is_ gaining weight, but Derek has yet to notice where it’s accumulating.

It doesn’t help that Stiles wears insanely baggy clothes, a hold over from high school Derek is trying to break because it make sit impossible to see if he’s all his hard work is paying off. Derek won’t lie, he likes the idea of seeing his handy work show it self on Stiles. He’s worked hard dammit!

Derek does start to notice things when Stiles hits 125 pounds. He sees a sway in his hips that wasn’t there before, and the seat of his pants don’t seem to hang off him like they used too. It’s not much, but it’s going somewhere Derek is really interested in going. 

They fuck. Of course they do, they’re two crazy college students in love with their own apartment. They fuck, and they fuck _often._ For a moment, Derek even considered their constant exercise the reason Stiles wasn’t putting weight on and withheld sex for nearly two weeks before proving it wasn’t that. 

Stiles putting on weight gave him more energy and as a result, they fucked more. Derek is all too happy to make Stiles come over and over again, and he’s just happy that Stiles’ ribs are starting to disappear. Derek loves Stiles, but his bones being apparent was a little unnerving. Derek is on his back, and above him Stiles rides him, it’s their favorite position. Derek grabs at Stiles’ ass, knowing the feeling is the flesh and the size he’s surprised to find that Stiles’ cheeks no longer feel small in his palms. Derek squeezes and confirms that yes, it _is_ bigger. 

Hell yeah. Derek gets Stiles to come four times that night. 

Week by week, Stiles weight goes up and Derek is thrilled. He starts keeping a journal of Stiles’ weight, and sometimes he even takes measurements. Through this, he finds that Stiles is gaining weight in one part of his body more than others: his ass. Stiles’ weight measures at a trim (SIZE) while just a few inches down his hips measure (SIZE). The difference is enough to make Derek ravish Stiles after each weigh in. 

Derek goes to buy more Superior Mass and even starts doubling the amount he puts in Stiles’ drink to try expediting Stiles’ gain. 

Soon, Stiles is gaining nearly 5 pounds a week all thanks to Derek’s hard work. Derek notices that it’s not only Stiles’ ass that is growing, but his thighs are thickening too. One morning, Stiles is running particularly late, so Derek goes to check on him and finds him struggling to get his jeans on over his thighs. He watches as Stiles jumps to get them up, and then wrestle the fabric to close enough to button them. 

“What’s going on?” Derek asks, and Stiles turns around to reveal that he is in fact struggling to get his pants on. 

“They’re too small!” The exclamation was a mix of joy and annoyance. 

“Then wear some sweats. No one will notice or care,” Derek answers. Stiles turns to look into the mirror putting his ass on display for Derek to see. The jeans are _tight_. Derek had noticed they were getting snug, but he didn’t imagine Stiles would actually outgrow them. He can’t help but think about how sexy Stiles looks with jeans nearly painted on. 

Stiles pulls the jeans down, revealing his boxers are also tight on him, the elastic cutting into the soft flesh that sits at his lower back, his muffin top. Derek moves his eyes up, to look at the rest of Stiles, his torso looks the same, the shirt he’s wearing is still baggy. He’s definitely not as skinny, but it was obvious where all his newly gained weight was being stored. 

Derek goes back out to the kitchen and adds two more oreos to Stiles’ snack bag. 

When Stiles comes out he’s wearing sweats and even those look tight across his ass. It makes Derek wonder what Stiles’ ass would look like in a Juicy tracksuit, _Juicy_ written across like it needed to be said. 

Derek uses his meticulously taken measurements to buy Stiles new pants. He staches the old ones in the closet, wanting to keep them for sentimental reasons and puts the new ones in the dresser. They’re a few sizes larger than what Stiles measure now, but Derek as a feeling Stiles will be filling them out in no time. 

Summer starts and Stiles makes a commitment to hang out with his dad for a week. Derek is of course invited along.

In the car, Derek is surprised to see how tight Stiles’ jeans sit on his thighs. Derek usually drives, and he’s usually focused on driving or their nightly debate about where to eat that he hasn’t noticed. It’s a shame really. Derek sees how much of the seat Stiles’ ass fills. His thighs flattening as he sits, spreading his legs in an almost obscene way. He’s grown a tiny little belly, one that just pokes out over the tops of his pants when he sits down, as soon as he stands it vanishes. 

When they get home, Stiles is thrilled to report to his dad that he’s gained 30 pounds since December break, putting him at just under 140 pounds. It’s been enough that Stiles’ doctor has stopped requesting monthly check ups. Stiles is wearing new new jeans, ones that disguise his fat ass easier than the ones back home. Sometimes Derek wonders if Stiles sees how his body has changed, he acts as though he doesn’t. He comments on his jeans getting tight, how they shrunk in the wash. Derek immediately replaces them with new ones and Stiles seems to forget all about it. 

Stiles does a twirl for his dad, ‘That’s great son!” John doesn’t seem to notice either. 

When they get back to school, Derek kicks up the dosage to three scoops. Derek had forgotten to pack the bottle and he couldn’t find the same brand in Beacon Hills. 

Stiles gets a job over the summer. Initially, it concerns Derek because he doesn’t want Stiles to do too much, at risk of losing the progress they’ve made. Luckily, Stiles gets a job at a local bakery, Derek can’t help but think how perfect it is. 

As the summer wears on, Stiles continues to gain, easily surpassing 150 pounds by the time school starts. 

Derek notices that Stiles whole legs are getting thicker, soon his calves start pressing on the material around them, making the skinny jeans prove Stiles is anything but. He continues to buy Stiles new pants, now nearly every month. 

Every once and awhile, he’ll slip and old pair into the drawer just to see Stiles jump to try to get them on, his ass jiggles and bounces and it’s glorious. Derek always ends up fucking Stiles good after those days. Grabbing on to his fat ass as Stiles either bounces on his dick or as Derek thrusts into Stiles. 

Other parts of Stiles slowly start to catch up. His little belly is the most notable, and soon Stiles decides to wear his pants over the little belly, which creates a wonderful little bulge in Stiles’ pants just under his fly and button. Stiles cheeks soften, growing round. His arms get soft and his chest--oh his chest. Derek noticed one night that Stiles’ pecs were giggling and it’s been a thrill to see his nipples puff up and push out. 

His shirts were _finally_ getting small. The hems were being pulled tight over his waist but now they were just small. Derek swaps new larger shirts in with the jeans. Stiles doesn’t even notice. 

The best part, to Derek at least, is that Stiles legs, thighs, and ass are getting so big that it’s causing him to develop a little waddle. The weight of his own body causing such a change in his walk. Derek’s favorite days of the week are weigh-in days, where Stiles proves to be 198 pounds.

Derek can’t believe it. Stiles rides him and Derek can’t even remember what it was like to have Stiles do this when he was so thin. The weight of Stiles on top of him was invigorating. The feeling of Stiles’ thighs bracketing him and keeping him down on the bed was _everything_. Derek never wanted it to end. 

They went to a restaurant to celebrate the end of the semester. Derek chose the place special because he knew the seats were small and that Stiles’ ass was big. It didn’t hurt that the place also had very good food. 

Stiles sits down in the chair, Derek feeling how small it feels for him and wondering how long Stiles will take it before he speaks up. The meal is delicious, and Derek orders for both of them, ording Stiles a meal that he knows is meant for two. They chat and talk, it’s always great when the semester ends and they don’t have to focus their entire conversations around schedules or homework. 

Derek isn’t surprised that Stiles eats everything on his plate, his appetite has been getting bigger. 

When it comes time to leave Derek stands with ease, placing down the tip for the waitress and looks to Stiles who remains seated. 

“Let’s go,” Derek says, knowing Stiles can’t. 

“I can’t,” Stiles admits, “I think I’m stuck!” Stiles pushes up on the arms of the chair, but the work is futile. 

“You’re chair must be smaller than mine,” Derek suggests. He moves to Stiles’ side of the table and offers Stiles a hand. Watching him struggle to get his ass out from the seat makes Derek flush and hot under the collar. Surely he _has_ to know how his body has grown?

Evidently not. 

“This is so messed up, why would your chair be a different size than my chair?” Stiles wiggles in the seat. Derek finally offers to hold the chair down so Stiles can stand. He also skips his hands into the space between Stiles’ ass and the arms to maybe allow less friction to keep him in place. 

Stiles stands successfully. On their way out, Stiles suggests that the restaurant make sure all the chairs are the same size. The hostess just looks confused as Stiles waddles out with Derek’s hand wrapped around his waist. 

The next time Derek and Stiles see John is during December break. Derek makes sure to pack his secret weapon this time. Nearly a year ago, Stiles weighed a meager 108 pounds. When Stiles waddles into his dad’s house, he weighs 213. Derek watches Stiles hug his dad, and notices that the size of Stiles’ legs keep him a small ways away from his dad, it’s thrilling. John doesn’t comment when Stiles announces how much he’s gained. He does so with such wonder and awe like he can’t believe he’s done so well. 

Derek has packed over 100 pounds on Stiles in the past year, he wonders how much he can do in the next one. Who knows, maybe he’ll make John a morning shake too, he’s looking a bit thin these days. 

Derek orders a palette of Superior Mass for the new year. 

**Author's Note:**

> I hope you enjoyed it! Please consider leaving a comment and a kudos! 
> 
> I'm working hard on the next entry, but I am running out of ideas, so if you have a prompt you've been thinking about feel free to drop it in my [Tumblr inbox!](https://modestfuckup.tumblr.com/ask)
> 
> More about [Get Beached 2020](https://get-beached.tumblr.com/)


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